Amorphous, synthetic silica prepared according to the gel process and set to controlled particle sizes by grinding silica gel, has for a long time served as an effective anti-blocking agent for polyolefin films (polyethylene, polypropylene, ethylene copolymers). Such silicas are known, for example, by the trade name SYLOBLOC.RTM.. Despite the good efficacy and the acceptability with regard to health, these silicas display disadvantages during handling and incorporation. Thus, during handling unpleasant dust forms, which moreover leads to losses. The disadvantages upon incorporation can negatively influence the film quality.
Precipitated silica, wet-chemically prepared (e.g. from acid and alkali silicate solution) is also used as an anti-blocking agent. Such types of silica likewise display the afore-mentioned handling and incorporation disadvantages.
In most cases, silica is incorporated using screw or internal kneaders. Here, use is often made of the master-batch technique, i.e. 5, 10, or 20 wt-% silica is incorporated into the plastics material. Because of the structure of silica, the melt viscosity of the plastics material rises drastically depending on the quantity of silica. This is also shown upon comparing the MFI (melt-flow index) of plastics material without silica and plastics material with silica. Plastics material with silica displays a considerably lower MFI.
However, the increase in the melt viscosity upon processing/incorporating silica leads to increased pressures in the extruder and to a reduction in output. Furthermore, because of the increase in the melt viscosity, under optimum production conditions only approximately 20 to 25 wt-% silica can be incorporated into the plastics material. Larger quantities, for example up to approximately 30 wt-%, are admittedly possible under certain circumstances, but then a reduced output with unsatisfactory distribution of the silica particles and with a further lowering of the melt-flow index must be expected. Too low a melt-flow index of the masterbatch (plastics material with silica) means that the latter is not properly distributed upon processing with film plastics. Thus, inhomogeneities occur in the film (flow lines, gels, silica agglomerates). This is, for example, the case if a masterbatch (plastics material with 20 wt-% silica) with a MFI of 1.0 is incorporated into a high-pressure polyethylene film raw material with a MFI of 2, because this masterbatch is not homogeneously distributed in the film raw material.
The afore-mentioned disadvantages during handling involve the major dust formation of the silicas which are processed with bulk densities of approximately 80 to 250 g/l. The dust which occurs when opening bags or when pouring silica into hoppers, mixers or containers is not only extremely unpleasant for the personnel, but also means a loss of silica for the user.